1. Field of the Invention
The present invention relates to a reflection-type optical device used for an optical communication system, such as a reflection-type circulator, a reflection-type isolator or a reflection-type optical switch.
2. Description of the Related Art
As optical devices used for an optical communication system, there are an optical circulator, an optical isolator and an optical switch. Many structures are known for the optical circulator, the optical isolator and the optical switch. As compared with a transmission type, according to a reflection-type optical circulator, optical isolator, or optical switch in which an optical fiber is disposed at only one side and a reflecting plate is disposed at the other side, the accommodation space of the optical fiber in a case where the reflection-type device is disposed in an apparatus can be made small, and therefore, the reflection type is effective in miniaturization of the whole apparatus.
FIGS. 34A and 34B show a structure of a conventional reflection-type optical circulator disclosed in patent document 1 (U.S. Pat. No. 5,471,340). As shown in FIGS. 34A and 34B, this reflection-type optical circulator includes three pairs of optical fibers 100 and lenses 102, a birefringent plate 104, two ½ wavelength plates 106 and 107, a Faraday rotator 108, a birefringent plate 110, a Faraday rotator 112 and a reflecting mirror 114. In the structure shown in FIGS. 34A and 34B, except the reflecting mirror 114 and the lens 102, three kinds and six optical elements (the two birefringent plates 104 and 110, the two ½ wavelength plates 106 and 107, and the two Faraday rotators 108 and 112) are required, and the element structure of the reflection-type optical circulator becomes complicated. Thus, there arises a problem that it becomes difficult to miniaturize the reflection-type optical circulator and to reduce the cost thereof. Besides, in the structure shown in FIGS. 34A and 34B, also when a light beam having passed through the birefringent plate 104 as an ordinary ray is reflected by the reflecting mirror 114 and is returned, it passes through the birefringent plate 104 as the ordinary ray. On the other hand, also when a light beam having passed through the birefringent plate 104 as an extraordinary ray is reflected by the reflecting mirror 114 and is returned, it passes through the birefringent plate 104 as the extraordinary ray. Since a light path length is difference between the case of passing through as the ordinary ray and the case of passing through as the extraordinary ray, in this structure, a value of polarization mode dispersion (PMD) does not become 0 but becomes large.
FIGS. 35A and 35B shows another conventional reflection-type optical circulator disclosed in the patent document 1. As shown in FIGS. 35A and 35B, this reflection-type optical circulator includes three pairs of optical fibers 100 and lenses 102, a birefringent plate 104, a Faraday rotator 108, two birefringent plates 110a and 110b, a Faraday rotator 112, and a reflecting mirror 114. In the structure shown in FIGS. 35A and 35B, although the element structure becomes simpler than the structure shown in FIGS. 34A and 34B, there arises a problem that the PMD value does not become 0 because of the same reason as the above.
FIG. 36 shows a structure of a conventional reflection-type optical circulator disclosed in patent document 2 (U.S. Pat. No. 5,930,422). As shown in FIG. 36, this reflection-type optical circulator includes three optical fibers 100, a birefringent plate 104, four ½ wavelength plates 106 (only two are shown in FIG. 36), a Faraday rotator 108, a birefringent plate 110, a lens 102 and a reflecting mirror 114. In the structure shown in FIG. 36, except the reflecting mirror 114 and the lens 102, three kinds and seven optical elements (the two birefringent plates 104 and 110, the four ½ wavelength plates 106 and the one Faraday rotator 108) are required, and the element structure of the reflection-type optical circulator becomes complicated. Thus, there arises a problem that it becomes difficult to miniaturize the reflection-type optical circulator and to reduce the cost thereof. Besides, because of the same reason as the above, there arises a problem that the PMD value does not become zero.
FIGS. 37A and 37B show a structure of a conventional reflection-type optical circulator disclosed in patent document 3 (U.S. Pat. No. 6,111,695). As shown in FIGS. 37A and 37B, this reflection-type optical circulator includes a birefringent plate 104, a birefringent plate 105, two Faraday rotators 108a and 108b, a birefringent plate 110, two Faraday rotators 112a and 112b and a reflecting mirror 114. In this structure, a light beam having passed through the birefringent plate 104 as an ordinary ray passes through the birefringent plate 105 as an extraordinary ray, and when it is reflected by the reflecting mirror 114 and is returned, it passes through the birefringent plate 105 as the extraordinary ray, and passes through the birefringent plate 104 as the ordinary ray. On the other hand, a light beam having passed through the birefringent plate 104 as an extraordinary ray passes through the birefringent plate 105 as an ordinary ray, and when it is reflected by the reflecting mirror 114 and is returned, it passes through the birefringent plate 105 as the ordinary ray and passes through the birefringent plate 104 as the extraordinary ray. Thus, in the structure shown in FIGS. 37A and 37B, the PMD value becomes zero. Such combination of the two birefringent plates 104 and 105 is known as a Savart plate. The Savart plate is used as an element to give a lateral shift to two polarized components perpendicular to each other without causing a phase difference. However, in the structure using the Savart plate as shown in FIGS. 37A and 37B, except the reflecting mirror 114 and a lens (not shown), seven optical elements (the three birefringent plates 104, 105 and 110, and the four Faraday rotators 108a, 108b, 112a and 112b) are required, and the element structure of the reflection-type optical circulator becomes complicated. Thus, there arises a problem that it becomes difficult to miniaturize the reflection-type optical circulator and to reduce the cost thereof.
As described above, the conventional reflection-type optical device has at least one of the problems that the element structure becomes complicated so that the miniaturization and reduction in cost become difficult, and the PMD value does not become zero.